Infant and Child Mortality Related to Family Size, Birth Interval In less developed countries infant and child mortality is much higher in large families than in small ones. For example, in eleven villages of the Indian Punjab during 1955-58, 206 out of 1,000 children died during the first year of life in families in which the mother had given birth to seven or more living children. In families of only two children, the infant mortality was 116 out of 1,000. The difference in mortality rates was even larger for children between 1 and 2 years of ageó95 per 1,000 for the children in families of seven or more live births and 16 per 1,000 for two-child families. The same proportionate differences in mortality rates between children of small and large families are found in New York City, though the levels of mortality are very much lower. The effects of short birth intervals on infant and child mortality in low income families are painfully illustrated by data from these Punjabi villages. In 1955-58, 310 out of 1,000 children, born less than a year after a preceding child, died during the first 2 years of life. This mortality rate was 55 percent greater than that of children born between 3 and 4 years after a previous birth, and more than twice as high as the mortality rate among children born after an interval of more than 4 years. The proportional differences in deaths during the second year of life between the three groups of children were about twice as large as the differences during the first year, though the mortality rates were considerably lower. Family Size and Physical Development In low income countries the high mortality rates among children in large families, and in families with close birth intervals, are in part due to malnutrition. The greater the sibling number, the greater the likelihood of malnutrition among poor families. Studies of preschool children in Colombia, for example, show that 52 percent of the children in families in which there were five or more preschool children were seriously malnourished, whereas only 34 percent of children in families with only one preschool child were malnourished. In Thailand, of the children whose next youngest sibling was born within 24 months, 70 percent were malnourished; of those in families without a younger sibling, only 37 percent. Since growth is related to nutrition, it would be expected that the height and weight of children in large families would be smaller on the average than in small families. Even in high income countries the children of poor families are larger at any given age when the number of children in the family is small. For example, of 2,169 London day school students 11.25 years old, children from one-child families were about 3 percent taller and 17 to 18 percent heavier than children from families with five or more children.